Let’s Talk Center of Gravity

By Rupert Kosmala

Illustrations by the author

As seen in the October 2001 issue of Model Aviation.

In the 20 or more years since I took up aeromodeling again, I have noticed that many sport RC fliers view the center of gravity (CG) as something to be highly respected, a necessary evil, and definitely not something to be messed with. 

Many times I have suggested to my fellow modelers that moving the CG can greatly improve the way a model flies. 

Frequently the response is along the lines of, “the CG is perfect; it’s exactly where the plans say it should be,” “it’s at 30% and I’m not moving it,” or “the airplane is flying okay, so why mess with it?”

It is often accepted as a cast-iron fact that moving the CG will make the model uncontrollable. 

For those of you who are willing to experiment, following is an explanation of how you can tune the handling of your airplane by adjusting the position of the CG. 

The CG is the point where the total weight of the airplane acts. It lies inside the model, somewhere in the cockpit of the typical high- or low-wing sport aircraft.

Weight is always pulling the airplane straight down, regardless of the model’s attitude. On the other hand, the neutral point (NP) is where all the lift acts. The wing generates most of the lift, but the fuselage also contributes. 

When an airplane is stable in pitch, it will go to a nose-down attitude when the pilot stops controlling it—unless the engine is producing enough power to overcome the stability. 

In order for a model to be stable in pitch, the CG has to lie somewhere in front of the NP. The farther forward the CG, the more stable the model; the more rearward the CG, the less stable the model. 

Some airplanes are so stable (nose-heavy) that they are difficult to fly. At the other end, if the CG is too far back and gets behind the NP, the model is unstable and impossible to fly. 

As the CG moves closer to the NP, the airplane approaches neutral stability; it becomes neutrally stable when the CG is at the NP. 

When the pilot takes his hands off the sticks of a neutrally stable airplane, it will continue flying along the same path because there is no stabilizing force to make it change, until gravity eventually wins. 

In this condition it is not unstable, but it has to be “flown” all the time. There is a trade-off between stability and maneuverability, which is why Pattern airplanes are neutrally stable. (See the stability diagram.) 

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Kit and plans designers will show a CG position where the airplane will definitely be stable, and it is often assumed that the CG must not be moved from that spot. 

Modelers commonly do the “balancing at the fingertip” test, by placing the fingertips at some mystery point behind the leading edge and seeing if the model balances. 

Some modelers even insist that the model’s nose should dip slightly, “just to make sure.” Once an airplane is known to be a flier, this test is as good as meaningless. The fact that it flies is all the modeler really needs to know about the CG position. 

Being certain about the actual position of the CG is only important before a first flight. It is not necessary to know the exact position of the CG before starting the process of adjusting it to get the best flying characteristics. 

After the first flight, the CG position is often taken for granted. But wherever it is, that is the starting point. The following tells how to do it. 

Materials Needed: 

• Basement beam or similar, fitted with a hook or a nail. 

• String, which is long enough and strong enough to support your airplane. 

• A very small—roughly four inches long—and very lightweight spirit level, which is available at any hardware store. 

• Weights, such as pennies, quarters, or washers, depending on how much weight you will need. The weights should all be the same. 

• A plumb bob. 

Steps: 

1) Tie a loop at each end of the string and fit one loop over the spinner/propeller and the other over the fin/rudder and around the tail wheel if you have one, or you may need to insert a pin to stop the loop from sliding off. 

The string should be long enough to suspend the airplane roughly six inches off the ground. 

2) If necessary, wind the string around the nail to stop it from slipping, then place the spirit level on a flat, horizontal surface; the stabilizer is usually a good place. Adjust the string so that the model hangs level. (See diagram.)

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Hang the plumb bob from the same nail or hook, and adjust the bob string so that its point is 1/4 inch from the airplane. 

3) Let the airplane and the bob settle and stop swinging. Approach the model slowly so that you don’t create air movement to start it swinging again. With a fine felt-tip pen, very carefully mark the spot where the bob is pointing. 

4) Place some weights on the stabilizer to make the bob move 1/8 inch to the rear. Mark the spot. Add that amount of weight again, and the bob will move rearward the same 1/8 inch. 

Now you know how much weight to add to move the CG rearward 1/8 inch. 

Suspending the model this way will also show you if the aircraft is out of balance side to side. One wing will probably be dipping, showing that the model is out of balance. 

Putting the fin/rudder in the loop of the suspending string helps keep the airplane straight during these checks. 

Find some weights to add to the high, or light, wingtip to bring it level. You can use screws in the wingtip, lead stick-on weights (but they tend to come unstuck), or insert some 1/8-inch-diameter solder pieces into the tip and apply thin CA glue. 

When an airplane is out of balance laterally, the heavy wing will tend to drop. Adding aileron trim to straight it out will work only for one throttle setting. 

Slow down and the heavy wing will drop again; speed up, and the other wing will drop. Because the correcting force from the aileron varies with speed, the airplane will never fly straight at all speeds. 

Weight acts downward and never changes with speed. It is quite rare for an airplane to be balanced laterally; there are differences in materials, construction, and mufflers that help unbalance a model. 

Similarly, when the CG is in a very forward position, it is pulling the nose down. Trimming the elevator to apply a downward force on the other side of the CG will only be effective for one speed. 

A model so trimmed to fly straight and level at half throttle will climb when the throttle is opened and will dive when it is closed. 

A forward CG makes an airplane difficult to land because the balancing downward force of the elevator decreases as it slows down, requiring more up-elevator. Sometimes the elevator runs out of travel before the airplane has slowed enough, and that can make for heavy “carrier” landings. 

Moving the CG rearward in small steps will reduce the pitch stability in small steps, and that will let you creep up on the best CG position. 

As you add the weights to the tail and move the CG a little bit at a time, the airplane will not suddenly become difficult to fly, but will gradually feel lighter and more responsive. 

A nose-heavy dog of a model will gradually become more and more fun to fly and easier to land. (If your airplane is already very sensitive to elevator movement, you may want to do the opposite by adding some weights to the nose then gradually removing them.)

When your model becomes a bit too lively for comfort, take off that last weight from the tail and you will have found the best CG position for your style of flying. 

Where the CG is in terms of percentage of the wing chord is unimportant, but it may be interesting to learn how the right CG position for you compares the one given on the plan. 

If you end up with unsightly weights on the stabilizer, try moving the battery or servos to regain the new CG position and do away with the weights. 

That’s an easy job now that you know how to accurately locate the CG. 

The correct CG position is determined by how the airplane flies—not by a mark on a piece of paper. That’s only the starting point. 

Happy flying!